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1 Module 4a: Water Demand Robert Pitt University of Alabama And Shirley Clark Penn State - Harrisburg Approach How much water is going to be required? Where is the water going to come from? How is the water going to be delivered? How much water is going to be required? Should be able to answer this question at two levels Regional Level Estimate population growth Estimate water requirements for population Subdivision Level Estimate water requirements for planned development It is very difficult to make an accurate prediction, especially about the future. Niels Bohr How much water is going to be required? Water-using Sectors Agriculture Approximately 50% of withdrawn water is consumptively used Withdrawals typically are seasonal and may be inversely related to natural water availability Navigation Hydroelectric Power/Steam Electric Generation Manufacturing Process Waters Cooling Waters Natural Systems Instream flow requirements Lake levels Protection of fish and wildlife Cities and Other Communities Domestic (drinking, cooking, laundry, sanitation) Street cleaning Fire demand Landscaping Commercial Use Recreation
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Page 1: Water demand

1

Module 4a: Water Demand

Robert PittUniversity of Alabama

AndShirley Clark

Penn State - Harrisburg

Approach

• How much water is going to be required?

• Where is the water going to come from?

• How is the water going to be delivered?

How much water is going to be required?

Should be able to answer this question at two levels

Regional Level– Estimate population growth– Estimate water requirements for populationSubdivision Level– Estimate water requirements for planned development

It is very difficult to make an accurate prediction, especially about the future.Niels Bohr

How much water is going to be required?Water-using Sectors

• Agriculture– Approximately 50% of withdrawn water is consumptively used– Withdrawals typically are seasonal and may be inversely related to natural water

availability• Navigation• Hydroelectric Power/Steam Electric Generation• Manufacturing

– Process Waters– Cooling Waters

• Natural Systems– Instream flow requirements– Lake levels– Protection of fish and wildlife

• Cities and Other Communities– Domestic (drinking, cooking, laundry, sanitation)– Street cleaning– Fire demand– Landscaping– Commercial Use

• Recreation

Page 2: Water demand

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How much water is going to be required?Problem arises in determining allocation of water between sectors.

Trend in Freshwater Withdrawals by Water-Use Category.From: Water Supply and Pollution Control, Sixth Edition. Warren Viessman, Jr. and Mark J. Hammer. Addison-Wesley. 1998.

How much water is going to be required?Problem arises in determining allocation of water between sectors.

Trend in Freshwater Withdrawals by Water-Use Category.From: The Cartoon Guide to the Environment. Larry Gonick and Alice Outwater. HarperPerennial. New York, NY. 1996.

How much water is going to be required?How much water do certain water-using sectors need?!

Water Allowance for American Forces Fighting in France in WWI.• From: The Story of Man’s Quest for Water. Jasper Owen Draffin. The Garrard

Press. 1939.

10.0General Use at Stables

25.0Base Hospitals

10.0Rear Regions

5.5Field Hospitals

1.0Army after 3 Days, Men

7.5Animal, Drinking

0.5Advancing Army, Men

Gallons per Man or Animal per day

How much water is going to be required?How much water do certain water-using sectors need?!

Public Water Supply in Gallons Per Head Per DayFrom: Studies in Ancient Technology, Volume 2. R.J. Forbes. E.J. Brill Publishers, 1964.

120New York

55Munich

40Frankfort

20Leipzig

5712.0Glasgow

527.5Edinburgh

36.53.5Liverpool

335.5Manchester

35.510.03London

3Paris

150250300198Rome1936183518301823A.D. 10050 B.C.

Page 3: Water demand

3

How much water is going to be required?How much water do certain water-using sectors need?!

Water Use in North American CitiesFrom: Environmental Science and Engineering, J.G. Henry and G.W. Heinke, Prentice-Hall, 1989.

100175660TOTAL

1526100Other

2544160Industrial

1526100Commercial

4579300Domestic

GpcdLpcd

Percentage of Total Use (%)

Average Daily Consumption Per Person*

Use

Distribution of per capita water demand

(Chin 2006 Table 2.5)

Regional Level

• Amount of water needed in a certain area will depend on:– Cost of Water– Conservation Regulations– Environmental Protection Regulations– Cost and Volume of Water Reuse– Economic Conditions

• Availability of Funds• Availability of Wastewater Treatment• Individual Attitudes towards Conservation, Reuse, Cost

Regional Level

The amount of water used in a locality is directly related to the size of the population. Errors in projecting population changes affect water use projections as well.

Page 4: Water demand

4

Regional Level

• Population ChangesFor any time period:

∆ Population = Births – Deaths + Immigration –Emigration

Predicted through the use of population projections made by demographers

Review of Past Population ChangesMovement of United States Population, 1930 – 1979.

From: The Times Concise Atlas of World History. G. Barraclough, editor. Hammond and The London Times. 1982.

Review of Past Population ChangesMovement of United States Population, 1975 – 1980.

From: Environmental Science, Action for a Sustainable Future, 3rd Edition. D.D. Chiras. Benjamin/Cummings Publishers. 1990.

Review of Past

Population Changes

Page 5: Water demand

5

Estimate population growth

Engineers normally do not conduct these analyses, as this is in the realm of social scientists and economists. Too many uncertainties exist in local and regional social-economic conditions to allow simple extrapolations of past conditions for the future. As an example, many smaller suburban communities actively recruit different types of industries that could have vastly different water demands and impacts on the local population.

Two Growth Models Typically Used: Check past census data to see which is more appropriate:• Arithmetic Method

Assumes constant amount of change in the population over time

• Exponential Growth Assumes a rate of increase which is proportional topopulation.

Arithmetic Method

Pt = population at time t in futurePo = present populationKt = average change in population per unit time period

tPK

KtPPt

∆∆

=

+= 0

Exponential Method (Uniform Percentage Method)

Pt = population at time t in futurePo = present populationr = change in population per unit timeK′ = slope of the line of past population vs. time

(on semi-log paper)t = time for which population change is being forecast∆t = time of population projection

tKPPOR

ePP

t

rtt

∆+=

=

'lnln 0

0

Other Methods: Declining Growth Method• Assumes city has limiting population• Assumes that rate of growth is function of population deficit (difference

between saturation population and actual population)

Where Psat = saturation populationP = population at future time (or later year when calculating K′′)P0 = population at base yearn = time interval between successive censuses (time between

collection of P0 and P values for calculating K′′)

⎟⎟⎠

⎞⎜⎜⎝

⎛−−

−=

−−+= ∆

0

''00

ln1''

)1)((

PPPP

nK

ePPPP

sat

sat

tKsat

Page 6: Water demand

6

Other Methods: Declining Growth Method

From: Water Supply and Pollution Control, Sixth Edition. Warren Viessman, Jr. and Mark J. Hammer. Addison-Wesley. 1998.

Other Methods: Ratio Method

• Use population forecasts from professional demographers for the appropriate region and ratio the design area’s current population to the current population of the trend line

• Use the ratio calculated above to estimate the future population

• Method assumes that the area’s population changes by the same ratio as the region

Other Methods: Curvilinear Method

• Graphically estimate future population based on recorded growths of larger cities (use data for other cites from the point at which they reached the current population of the design city/area)

• For example:– City A, the city being studied, is plotted up to 1990, the

year in which its population was 51,000. City B reached 51,000 in 1950, and its growth is plotted from 1950 to 1990. Similarly, curves are drawn for cities C, D, and E from the years in which they reached A’s 1990 population. A’s growth curve is then projected by considering the recorded growth of the comparison cities.

Other Methods: Curvilinear MethodCity A, the city being

studied, is plotted up to 1990, the year in which its population was 51,000. City B reached 51,000 in 1950, and its growth is plotted from 1950 to 1990. Similarly, curves are drawn for cities C, D, and E from the years in which they reached A’s 1990 population. A’s growth curve is then projected by considering the recorded growth of the comparison cities.

Page 7: Water demand

7

Example using Arithmetic and Uniform Percentage Method

10.23327800284002020

9.98021600227002010

9.45712800142002000

9.30611000124001995

9.1909800113001992

ln Population Served

Population Served

Population Total

Year

Example using Arithmetic and

Uniform Percentage Method

Estimate water requirements for predicted population size

Varies greatly from city to cityHighly dependent on:

climateeconomic conditionssocial attitudes toward environmental protectionlocal conservation regulationstourism

etc.

Population Density Estimations

Guide to Population DensityFrom: Water Supply and Pollution Control, Sixth Edition. Warren Viessman, Jr. and

Mark J. Hammer. Addison-Wesley. 1998.

5 – 15Industrial Areas15 – 30Commercial Areas

100 – 1000Apartments30 – 100Multiple-Family Units5 – 35Single-Family Units

ResidentialNumber of Persons Per AcreArea Type

Page 8: Water demand

8

• Average per capita use of public water supplies in the U.S. averages about 183 gallons per day (U.S.EPA, 1990).

• However, this number can vary greatly between regions and between communities

Future Per Capita Estimates of Water UseProjected Consumption of Water for Various Purposes in the

Year 2000From: Water Supply and Sewerage, Sixth Edition. Terence J. McGhee. McGraw-Hill

Publishing Company. 1991.

100176.88TOTAL813.2Loss and Waste915.84Public

1526.4Commercial2442.24Industrial4479.2Domestic

Percentage of TotalGallons Per Capita/Day

Use

Factors Affecting Consumption of Water

• Metering↑ Consumption ↓• Quality↑ Consumption ↑• Pressure↑ Consumption ↑• Rates↑ Consumption ↕• Sewer Service↑ Consumption ↑• Income↑ Consumption ↑• City Size↑ Consumption ↑• Industries↑ Consumption ↑• Very cold↑ Consumption ↑• Very hot↑ Consumption ↑

Page 9: Water demand

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Subdivision Level: Estimate water requirements for planned development

• Many communities have master development plans which establish the allowable uses of various sub-areas: industrial, commercial, public, and residential.

• When such plans exist, they are the best point at which to begin since water consumption can normally be related to land use.

System Design• Can do estimates based on number and/or types of structures

in design area and using existing data.• Residential:

Residential Water ConsumptionFrom: On-Site Wastewater Treatment: Educational Materials Handbook. National

Small Flows Clearinghouse. West Virginia University, 1987.

10070TOTAL53Drinking/Cooking

2014Laundry/Dishes

3021Bathing/Personal Hygiene

4532ToiletPercentGallons

Daily Water Use Per PersonHome Uses

Residential Areas

• Easier to evaluate areas since anticipated population densities may be established from the residential classification.

• Once an estimate has been made of population density, the average and peak flows can be determined.

Page 10: Water demand

10

Residential System Design

• Water use often assumed to be 100 gallons/person/day.

From: The Cartoon Guide to the Environment. Larry Gonick and Alice Outwater. HarperPerennial. New York, NY. 1996.

Residential System Design

• Residential Water-Use ModelsWinter use (indoor) in metered and sewered areas (single-family)

Q1 = 234 + 1.451 V - 45.9 Pa - 2.59 IaWhere Q1 = estimated water use

V = market value of residence in $1000sPa = sum of water and sewer charges in $/1000 galIa = effective Nordin/Taylor bill difference evaluated

at average indoor use in $/billing period

Winter use (indoor) in flat rate areas with sewers (apartments)Q2 = 28.9 + 1.576 V - 33.6 Dp

Where Dp = number of persons in dwelling unit

Residential System Design

• Residential Water-Use ModelsWinter use (indoor) in flat-rate areas with septic tanks

(multifamily)Q3 = 30.2 + 39.5 Dp

Summer use (indoor and outdoor) in metered and sewered areas (eastern U.S.)Q6 = 385.0 + 2.876 V - 285.8 Ps - 4.35 Is + 157.77(B•MD)Where Ps = sum of water and sewer charges that vary

with outdoor use in $/1000 galIs = effective Nordin/Taylor bill difference

applicable to outdoor use in $/billing period

B = irrigable landscape per dwelling unit in acresMD = summer season moisture deficit in inches

Industrial Areas

• Water use is industry-specific• Highest industrial users of water usually do not

use publicly supplied water• For water main design, assume equivalent to

high-density residential zone• 20L/m2-day or 100,000gal/acre-day

Page 11: Water demand

11

Commercial Areas

• Also specific for use – being highest for hotels and hospitals

• Offices and retail sales facilities may range up to 90 L/m2-day or 1000,000 gal/acre-day for multi-story construction

• A reasonable average value for undefined commercial development is 40L/m2-day or 45,000 gal/acre-day applied to the land area actually covered by structures, not including parking lots or grassed areas.

Multi-Family Residential and Commercial Water Use (Based on the Residential Water Use Research Project of The Johns Hopkins University and the Office of

Technical Studies of the Architectural Standards Division of the Federal Housing Administration, 1963):

From: Water Supply and Pollution Control, Sixth Edition. Warren Viessman, Jr. and Mark J. Hammer, Addison-Wesley. 1998.

0.084 gpd/ft24.89206,00041,400490,000 ft2Commer. Credit

Office Buildings

407 gpd/room307 gpd/room

1.39156,000112,000126,000

275 rooms410 rooms

HotelsBelvedereEmerson

69 gpd/unit1.8921,60011,400166 unitsMotel

156 gpd/unit3.4111,7003,43022 unitsApartment Building

Miscellaneous Residential

Average Annual Demand Per Unit

Ratio, Maximum Hourly to Average Annual

Maximum Hourly Demand Rate (gpd)

Average Annual Demand (gpd)

Break-down of Water Use

Multi-Family Residential and Commercial Water Use (Based on the Residential Water Use Research Project of The Johns Hopkins University and the Office of Technical Studies of the Architectural Standards Division of the

Federal Housing Administration, 1963):From: Water Supply and Pollution Control, Sixth Edition. Warren Viessman, Jr. and Mark J.

Hammer, Addison-Wesley. 1998.

0.18 gpd/ft226,000145,000 ft2Hillendale

0.15 gpd/ft22.5089,90035,500240,000 ft2Towson Plaza

Shopping Centers

0.070 gpd/ft22.5871,80027,000389,000 ft2State Office Building

0.082 gpd/ft25.0174,70014,900182,000 ft2Internal Revenue

Office Buildings

Average Annual Demand Per Unit

Ratio, Maximum Hourly to Average Annual

Maximum Hourly Demand Rate (gpd)

Average Annual Demand (gpd)

Break-down of Water Use

Multi-Family Residential and Commercial Water Use (Based on the Residential Water Use Research Project of The Johns Hopkins University and the Office of Technical Studies of the Architectural Standards Division of the

Federal Housing Administration, 1963):From: Water Supply and Pollution Control, Sixth Edition. Warren Viessman, Jr. and Mark J.

Hammer, Addison-Wesley. 1998.

472 gpd/lift26.512,5004721 liftService Station

330 gpd/car/hr of capacity

9.4675,0007,93024 cars/hr capacity

Car Wash

184 gpd/washer

251 gpd/ washer equiv.

6.85

6.45

12,600

16,200

1,840

2,510

Ten 8-lb washersEqual to 10 8-lb washers

Laundries Laundromat

Commercial

Miscellaneous Commercial

Average Annual Demand Per Unit

Ratio, Maximum Hourly to Average Annual

Maximum Hourly Demand Rate (gpd)

Average Annual Demand (gpd)

Break-down of Water Use

Page 12: Water demand

12

From: Water Resources Engineering, 1st Edition. Larry W. Mays, John Wiley & Sons, Inc. 2001. (Table 11.1.4 page 346)

63.910.3L/day/m2Laundry

20225.2L/day/m2Medical offices

1600503L/day/bedNursing homes

34501310L/day/bedHospitals

55L/day/person served

Night clubs

63291.6L/day/seatRestaurants

40501020L/day/stationBeauty shops

1470207L/day/barber chair

Barber shops

63.19.1L/day/m2Motels

17.610.4L/day/m2Hotels

Peak UseAverage UseUnits

From: Water Resources Engineering, 1st Edition. Larry W. Mays, John Wiley & Sons, Inc. 2001. (Table 11.1.4 page 346)

946401L/day/studentResidential colleges

503503L/day/alleyBowling alleys

84117L/day/memberGolf-swim clubs

17.80.5L/day/memberChurches

1280194.7L/day/inside m2Car washes

1020136L/day/m2Bus-rail depot

45825.1L/day/studentHigh schools

18620.4L/day/studentElementary schools

114.3L/day/sales m2Retail space

26588.4L/day/m2Laundromats

Peak UseAverage UseUnits

From: Water Resources Engineering, 1st Edition. Larry W. Mays, John Wiley & Sons, Inc. 2001. (Table 11.1.4 page 346)

203006780L/day/establishmentFast food restaurants

1640821L/day/occupied unitApartments

128010.2L/day/inside m2Service stations

12.612.6L/day/seatTheaters

14.45.8L/day/m2Old office buildings

21.23.8L/day/m2New office buildings

Peak UseAverage UseUnits

From: Water Resources Engineering, 1st Edition. Larry W. Mays, John Wiley & Sons, Inc. 2001. (Table 11.1.5 Page 347)

≤ 20L/minuteBathroom faucet

10 – 20 L/minuteRunning garbage disposal

20 – 40 LWashing dishes in filled sink

≤ 20L/minuteWashing dishes with tap running

40 –100L/loadWater-saver dishwasher

50 –120 L/loadDishwasher

≤ 20L/minuteNonstop running toilet

≥ 150L/daySilent leak

≤ 6L/flushUltra volume toilet

10 – 30 L/flushStandard toilet

130 – 270L/loadWashing machine

Average UseUnits

Page 13: Water demand

13

From: Water Resources Engineering, 1st Edition. Larry W. Mays, John Wiley & Sons, Inc. 2001. (Table 11.1.5 Page 347)

2300L/hour¾-Inch diameter hose

1900L/hour5/8-Inch diameter hose

1100L/hour½-Inch diameter hose

1 – 10 L/hourOne drip-irrigation emitter

110 – 910 L/hourStandard sprinkler

7600 – 16000L/monthWatering 750-m2 lawn

100 – 300LFilling bathtub

6 – 11 L/minuteLow-flow shower head

20 – 30 L/minuteShower head

8LBrushing teeth

Average UseUnits

From: Water Resources Engineering, 1st Edition. Larry W. Mays, John Wiley & Sons, Inc. 2001. (Table 11.1.5 Page 347)

300 – 1200L lost/monthCovered pool

3000 – 11000+L lost/monthUncovered pool

≥ 60L/20 minutesWashing car with pistol-grip faucet

400 – 800 L/20 minutesWashing car with running water

Average UseUnits

Given average annual consumption rates, still need to estimate peak demand because

• Water use varies during the day• Water use varies from day to day• Water use varies weekly and seasonally

Diurnal curves for different user categories

(Walski, et al. 2001 figure 4.8)

Page 14: Water demand

14

Daily Water-Use Patterns in Residential

Area: Maximum Day and

Minimum Day

From: Water Supply and Pollution Control, Sixth Edition. Warren

Viessman, Jr. and Mark J. Hammer, Addison-

Wesley. 1998.

Daily Water Use Patterns: Maximum Day and Winter DayFrom: Water Supply and Pollution

Control, Sixth Edition. Warren Viessman, Jr. and Mark J. Hammer,

Addison-Wesley. 1998.

Typical daily cycles in water demand

(Chin 2006 Figure 2.24)

Water and Wastewater Flow (No Lawn Irrigation)From: Water Supply and Pollution Control, Sixth Edition. Warren Viessman, Jr. and Mark J. Hammer,

Addison-Wesley. 1998.

Page 15: Water demand

15

Peak Water Use Estimation

• Consumption rate for max day = 180% of the annual average daily consumption

• Consumption rate for max week = 148% of the annual average daily consumption

• Consumption rate for max month = 128% of the annual average daily consumption

• Consumption rate for max hour = 150% of the max day, or 270% of the annual average daily consumption

Peak Water Use Estimation: Estimation of Average Daily Rate Based on a Maximum Time Period

Goodrich Formula:• Estimates maximum demand (expressed as daily water demand based

on time period for which maximum water demand is desired) for community when given annual average per capita daily water use rate:

where p = percentage of average annual rate (volume/day) used in period of time of interest

t = length of period for which peak demand is required (days) (valid time periods – 2 hours to 360 days)

• **Daily rate based upon a maximum hour is approximately equal to 150 percent of average annual daily rate.

10.0180 −⋅= tp

Peak Hourly Demand vs. Number of

Dwelling UnitsFrom: Water Supply and Pollution Control, Sixth

Edition. Warren Viessman, Jr. and Mark J. Hammer. Addison-Wesley. 1998.

Typical demand factors

(Chin 2006 Table 2.6)

Page 16: Water demand

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Fire Demand(Equation from the National Board of Fire Underwriters

for communities with less than 200,000 people)

Where Q = demand in gallons/min P = population in thousands

NOTE: Used when calculating fire demand for sizing reservoirs! This equation is used for the community as a whole (averaged conditions, not for water distribution system pipes!).

( )PPQ 01.011020 −=

Fire Demand: Sizing for Communities

• Community of 175,000 people. Calculate fire demand:

P = 175

Or rounding to 12,000 gpm or 45,000 L/min

( )( )

min/314,44708,1117501.011751020

01.011020

LgpmQQ

PPQ

==−=

−=

Fire Demand(from the Insurance Services Office)

where Q = fire demand in gallons/minuteA = total floor area excluding basements, ft2

C = coefficient for construction materialsC = 1.5 for wood frameC = 1.0 for ordinary constructionC = 0.8 for non-combustible constructionC = 0.6 for fire-resistant construction

ACQ 18=

For this equation, flow should be:•Greater than 500 gpm, but•Less than 6,000 gpm (single-story structure); 8,000 gpm (single building); 12,000 gpm (single fire)

Fire Demand(from the Insurance Services Office)

For example:• Building is ordinary construction, with an area on each floor of

1000 ft2 (no basement), and six stories.

or rounding to 1,400 gpm or 5,300 L/min

( )( )min/276,5394,1

6/21000)0.1(18

18

LgpmQstoriesstoryftQ

ACQ

==

=

=

Page 17: Water demand

17

( )iiii PXOCNFF +=In metric units (AWWA 1992):

C is the construction factor based on the size of the building and its construction,

O is the occupancy factor reflecting the kinds of materials stored in the building (ranging from 0.75 to 1.25), and

(X+P) is the sum of the exposure factor and the communication factor that reflect the proximity and exposure of the other buildings.

ii AFC 220=C (L/min),

A (m2) is the effective floor area, typically equal to the area of the largest floor plus 50% of all other floors,

F is a coefficient based on the class of construction

Construction coefficient, F

(Chin 2006 Table 2.7)

Occupancy factors, Oi

(Chin 2006 Table 2.8)

Needed fire flow for one- and two-family dwellings

(Chin 2006 Table 2.9)

Page 18: Water demand

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(Walski, et al. 2001)

Required fire flow durations

(Chin 2006 Table 2.10)

Example 2.15 from Chin 2006

Estimate the flowrate and volume required to provide adequate protection to a 10-story noncombustible building with and effective floor area of 8,000 m2.

( )iiii PXOCNFF += ii AFC 220=

( ) min/000,1680008.0220 2 LmCi ==

( )( )( ) min/000,174.175.0min/000,16 LLNFFi ==

( )( ) 36 080,41008.4min/604min/000,17 mLxhrhoursLV ===

The construction factor is calculated as (F=0.8 for class 3 noncombustible construction and the floor area is 8,000 m2):

The occupancy factor C is 0.75 (C-1 noncombustible) and the (X+P) is estimated using the median value of 1.4. Therefore, the required fire flow is:

The flow must be maintained for a duration of 4 hours, and the required volume is therefore:

Fire DemandAdd fire demand to maximum rate

Minimum Flows• Usually 25 – 50% daily average• Less important but is considered in the design

of pumping plants• Depends largely on loss and waste, and night

industrial use

Page 19: Water demand

19

Where is the water going to come from?

Design periods and capacities in water-supply systems

(Chin 2006 Table 2.11)

Page 20: Water demand

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Regional Planning Level

• Determine if current sources are adequate based on predicted growth

• Identify potential new sources• Develop conservation strategies • (reducing system loss, education, regulations,

etc.)

Must consider competing uses of resource

Irrigation for AgricultureIndustry Withdrawals Sustaining NavigationCities DownstreamSustaining Natural Systems

in-stream flow requirementsminimum lake levelsprotection of fish and wildlife

Hydroelectric Power/Steam Electric GenerationRecreation

Subdivision Level

• Check with municipalities or county to determine the water provider for the area in which you are working

• How you proceed will depend on the water provider

Birmingham Water Works Board

• Developer provides development plan to BWWB

• BWWB designs distribution system and determines cost of construction

• Developer pays cost up-front• BWWB constructs distribution system

Page 21: Water demand

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Shelby County

• Developer contracts with engineer to design distribution system

• Developer provides design to Shelby county• Shelby county reviews and approves• Developer constructs system• Shelby county inspects along the way• After completion and final inspection, Shelby county

takes over system as part of their own

How is the water going to be delivered?

Designing the distribution system

Methods of Water Distribution

• Gravity– Dependable– Source of supply must be located well above the city– High-pressure demand for fire-fighting may require

pumper trucks

• Pumping without Storage– Least Desirable– Pressures vary substantially with variations in flow– Provides no reserve if power failure

Methods of Water Distribution, cont.

• Pumping with Storage– Most common– Water supplied at approximately uniform rate– Flow in excess of consumption stored in elevated tanks– Tank water provides flow and pressure when use is high

• Fire-fighting• High-use hours• Flow during power failure

– Storage volume throughout system and for individual service areas should be approximately 15 – 30% of maximum daily rate.

Page 22: Water demand

22

Water Distribution System Components

• Pumping Stations• Distribution Storage• Distribution System Piping

Other water system components include water source and water treatment

Types of Distribution System

• Branch– No circulation– Has terminals and dead ends

Types of Distribution System

• Grid– Furnishes supply from more than one direction– In case of water main break, very few people are

inconvenienced

Types of Distribution System, cont.

• Combination– Grids cover most of the system– Terminals are side streets and houses only

Most common system in urban areas

Page 23: Water demand

23

Looped and branched networks after network failure

(Walski, et al. 2001 figure 1.2)

The Pipe System

• Primary Mains (Arterial Mains)– Form the basic structure of the system and carry

flow from the pumping station to elevated storage tanks and from elevated storage tanks to the various districts of the city

• Laid out in interlocking loops• Mains not more than 1 km (3000 ft) apart• Valved at intervals of not more than 1.5 km (1 mile)• Smaller lines connecting to them are valved

The Pipe System, Cont.

• Secondary Lines– Form smaller loops within the primary main

system– Run from one primary line to another– Spacings of 2 to 4 blocks– Provide large amounts of water for fire fighting

with out excessive pressure loss

The Pipe System, Cont.

• Small distribution lines– Form a grid over the entire service area– Supply water to every user and fire hydrants– Connected to primary, secondary, or other small

mains at both ends– Valved so the system can be shut down for repairs– Size may be dictated by fire flow except in

residential areas with very large lots

Page 24: Water demand

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Pipe sizes in Municipal Distribution Systems

• Small distribution lines providing only domestic flow may be as small as 4 inches, but:– < 1300 ft in length if dead ended, or – < 2000 ft if connected to system at both ends.

• Otherwise, small distribution mains > 6 in • High value districts – minimum size 8 in• Major streets – minimum size 12 in

• Fire-fighting requirements> 150 mm (6 in.)

• National Board of Fire Underwriters> 200 mm (8 in.)

Velocity in Municipal Distribution Systems

(McGhee, Water Supply and Sewerage, 6th Edition)• Normal use < 1m/s, (3 ft/s)• Upper limit = 2 m/s (6 ft/s) (may occur in

vicinity of large fires)

(Viessman and Hammer, Water Supply and Pollution Control, 6th Edition)

1< V < 1.7 m/s (3 < V < 5 ft/s)

Pressure in Municipal Distribution Systems(American Water Works Association)

AWWA recommends normal static pressure of 400-500kPa, 60-75lb/in2

- supplies ordinary uses in building up to 10 stories- will supply sprinkler sytem in buildings up to 5

stories- will provide useful fire flow without pumper trucks- will provide a relatively large margin of safety to

offset sudden high demand or closure of partof the system.

Pressure in Municipal Distribution Systems(McGee)

• Pressure in the range of 150 – 400kPa (20 to 40 lb/in2) are adequate for normal use and may be used for fire supply in small towns where building heights do not exceed 4 stories.

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Minimum acceptable pressures in distribution systems

(Chin 2006 Table 2.12)

Typical elevated storage tank

(Chin 2006 Figure 2.27)